1. Field of the Invention
This invention relates to polymers containing aromatic imide linkages, commonly referred to as polyimides (PI). Specifically, the invention relates to the production of articles made from polyimides in shorter times and by simpler methods than has been previously possible.
2. Description of the Related Art
Polyimides are classified as condensation polymers produced from reaction of bifunctional diamines and bifunctional or tetrafunctional carboxylic acid anhydrides. The imide structure --CO--NR--CO is found in the backbone of the polymer chain. Aromatic, heterocyclic imides are known for their exceptional mechanical characteristics and excellent chemical and high temperature resistances. Many types of aromatic polyimides have been created, depending on the amine and acid chemical structures. Pyromellitic dianhydride and di-(4-aminophenyl) ether yields Dupont's commercial polyimide product, Vespel.TM.. Trimellitic anhydride and an amine such as o-bisaniline or trimellitoyl chloride and methylene dianiline yields Amoco's commercial polyamide-imide product, Torlon.TM.. The self-condensation of trimellitic acid isocyanate yields Upjohn's P2800.TM.. Condensation of bisphenol and dinitrobisimide yields General Electric's commercial polyetherimide product, Ultem.TM.. Bismaleimides are produced by reacting maleic anhydride with diamines. Ciba-Geigy's P13N.TM. and Rhone-Poulenc's Kinel.TM. are commercial examples of reactive bismaleimides. Aromatic polyester, anhydrides and certain bismides yield polyesterimides, such as Dynamit Nobel's Icdal Ti40.TM.. Aliphatic polyester anhydrides yield polyarylates. Other pure polyimides are Lenzing P84-HCM.TM., Mitsui-Toatsu Aurum .TM., Furon Meldin 3000.TM., National Starch Thermid.TM. and NASA Langley LaRC TPI series.
Polyimides are generally characterized by great backbone rigidity that yields exceptional high temperature strength and thermal stability. They are frequently used for high temperature applications [&gt;230.degree. C.]. As a result, they are used as bearing seals, gaskets, piston rings, pressure discs, sleeves, sliding rods, valve shafts, automotive and appliance gears, brake components, cams, exhaust valves and stems, copier gears and so on. They are also characterized by extreme processing difficulty. Many polyimides cannot be molded by conventional thermoplastic processing techniques. Others require very high processing temperatures [300.degree. C. to 400.degree. C. or more] and pressures [1000 atmospheres or more]. As a result, polyimides are usually processed as powders using powder metallurgical techniques such as hot isostatic pressing, direct forming and compaction-sinter-forging or by compression molding where the powder is placed in a heated mold, heated to a forming temperature, compressed into shape, then cooled in the mold while under pressure. Many parts are machined from billets. Alternately, the polymers are dissolved in a suitable solvent such as n-methylpyrrolidone. Small molded parts, thin films and carbon-fiber laminates are produced from these solutions. With polyamide-imide polymer, the polymer is injection molded or extruded while it is not fully imidized. The molded or extruded part is then reacted at elevated temperature but in the solid state to a fully imidized state. Fully imidized polyamideimide cannot be molded using conventional plastic process techniques. It is believed that amorphous polyimides require decomposition temperatures higher than their melt processing temperatures and that the melt processing temperatures must be at least 50.degree. C. to 100.degree. C. above the glass transition temperatures of the polymers.
The limited tractability of polyimides with conventional plastics processing equipment has restricted the development of many applications. It has now been found that polyimides, including reactive polyamide-imides, can be molded into articles of commerce by first rapidly heating the polymer powder without shear to proper molding temperature, then compressing the powder into the desired shapes using conventional compression molding techniques. In a similar fashion, polyimide powder can be first rapidly heated without shear to a proper molding temperature, then ram extruded into desired profiles. The method of rapid heating involves mechanically impacting and imparting kinetic energy to the powder particles in a batch-wise fashion, while restricting and controlling the transfer of energy from the particles to the chamber holding the powder particles. One device that is used as a powder heater is shown by Goeser et al, U.S. Pat. No. 3,266,738, published Aug. 16, 1966. This patent describes a high intensity mixer available on the market today, under the trade mark Gelimat, made by Draiswerke GmbH. The device includes a plurality of blades that rotate about a horizontal axis within an enclosed jacketed container. The rotational speed of the blades is controlled with tip speeds of up to 50 meters/second possible. In Crocker et al. U.S. Pat. No. Re. 33,214, this device is coupled with an infrared detector that continuously monitors the powder temperature to ensure adequate temperature control while the constant rotational speed of the blades exceeds 25 meters/second. In Crocker et al. U.S. Pat. No. 4,420,449, published Dec. 13, 1983, the device so equipped is used to thermokinetically heat polytetrafluoroethylene (PTFE) powder at constant tip speeds of at least 30 meters/second prior to forming. In Crocker U.S. Pat. No. 4,272,474, published Jun. 9, 1981, the device so equipped is used to thermokinetically heat ultrahigh molecular weight polyethylene (UHMWPE) powder at constant tip speeds of at least 18 meters/second prior to forming.
It has surprisingly been found that it is not possible to mechanically heat polyimide powders only using either continuous monitoring of powder temperature or constant tip speeds of 18 meters/second, 25 meters/second, or 30 meters/second or any other constant tip speeds. It has further surprisingly been found that the heating rate of polyimide powders can be altered over wide ranges with careful balance of the chamber wall temperature and the speed of the mechanical heating device. This control allows for uniform heating of powder charges of widely varying rates. It further allows for controlled heating of powder charges such that partially imidized powder can be solid state reacted to full imidization at temperatures a few degrees below the molding temperatures. It further allows for degassing, devolatilizing and dewatering of polyimides that might contain byproducts of the condensation polymerization process including dissolved gases, volatile liquids, water, acetic acid, hydrochloric acid or other low boiling simple molecules. And it is further surprisingly been found that certain polyimides that are known to be unmoldable by any conventional plastic processing technique, can be mechanically heated to very high temperatures, typically in excess of their degradation temperature using a mechanical heating device with very hot chamber wall temperatures, in excess of 150.degree. C., and that these polyimides, when discharged from the mechanical heating device, can be molded into useful monolithic articles using conventional compression molding techniques with molds having temperatures 10.degree. C. to as much as 100.degree. C. below the glass transition temperatures of the polyimide. And it is further surprisingly been found that the reground powders of certain polyimides known to have a certain degree of crosslinking, to be considered as having thermosetting characteristics and to otherwise be considered as having no reprocessing capability, can be mechanically heated in a mechanical heating device with very hot chamber walls and the discharge molded into useful monolithic articles using conventional compression molding techniques.
After the heated polyimide powder is discharged from the mechanical heating device, the charge is transferred to a mold or die contained in conventional plastics processing equipment such as a compression molding press, a transfer molding press, a ram extruder, a forging press or other hydraulically, pneumatically or mechanically assisted press whereupon pressure is applied that is adequate to force the charge into the desired shape in the mold or die that has a temperature about 10.degree. C. to 100.degree. C. or more below the glass transition temperature of the polymer. The pressure is held against the shaped charge until the charge retains the general shape of the mold cavity. Articles made in this fashion have essentially no porosity, essentially no internal voids, essentially no cracks and mechanical properties that are equal to or greater than those of polyimides fabricated in more traditional ways.